1. Field of the Invention
The present invention relates to a heating system for a fluid pipe system, in particular a system of this kind in a motor vehicle, in which at least one electric heating element is assigned to at least one fluid line, said heating element being supplied with an electric operating voltage for applying a specified heating output to the fluid line.
2. Related Technology
For illustrative examples of known technologies relating to heating systems for fluid pipe systems, see patent documents DE 41 35 082 C1, WO 2007/073286 A1 and EP 985 908 A1.
Heatable fluid-pipe systems are often used in motor vehicles in particular, especially for fluid media which, due to their freezing point, tend to freeze at relatively high ambient temperatures. Such freezing can impair certain functions. This applies, for example, to water lines for windshield wipers and, in particular to, the lines carrying an aqueous urea solution that is used as an NOx reduction additive in diesel engines with so-called SCR catalysts. For this reason, electric heating elements can be activated at low temperatures in order to prevent freezing, or in order to unthaw an already frozen medium.
Such fluid pipe systems—cf. in particular EP 1 985 908 A1 (FIG. 13, 14) and WO 2007/073286 A1—are normally comprised of at least one fluid line (pipe or hose line) with two line-connectors (plug-in connectors) at the ends. The fluid line has an electric heating element in the form of a heating-filament winding, for example, that runs helically along the length of the line, and at least one of the line-connectors (WO 2007/073286 A1), or as the case may be each of the two connectors (EP 1 985 908 A1) is also provided with an electric heating element, particularly one in the form of a heating-filament winding. Normally, all of the heating elements are electrically connected in series and can be connected to a common power supply, or as the case may be supply voltage (cf. in particular EP 1 985 908 A1, FIG. 14a, 14b).
One problem that arises in such heating systems is that the fluid lines inside the pipe system generally display very different lengths, so that the effective heating resistance of the heating elements, and thus also the heating outputs, are correspondingly different. Particularly in the case of very short fluid lines (for example those with a length of less than 500 mm), the heating element that runs helically across the line can be so short that the heating resistance is too small as well. Greater resistance could be achieved by using a heating conductor with a smaller cross section, but that would result in mechanical problems. Then it would no longer be possible to produce reliable heating conductor connections (particularly crimp connections) due to the narrow cross-section and the resulting low mechanical strength.
As seen from the above, there is a need for a heating system of the cited type that simply and economically optimizes the electric heating output, or as the case may be, adjusts it to the existing conditions.
This is inventively achieved by means of the present invention, the principles of which are set out in each independent claim recited herein. Advantageous design characteristics are also set out in the dependent claims and in the description that follows.
In a first embodiment, the present invention reduces the operating voltage of the heating element relative to a supply voltage, in particular compared to the direct-current voltage of a motor vehicle's electric system (a normal battery voltage of, for example, 9-16 V or 20-32 V). This can essentially be done in any way chosen.
A series resistor can also be connected in series upstream of the heating element. The series resistor can also be configured as a heating conductor. This could also be, for example, a so-called cement resistor. The series resistor can be accommodated in a separate housing, or also in the region of a line-connector housing. Furthermore, it is also possible to arrange the series resistor in the region of a line-connector, in particular inside an encapsulation.
In another alternative, a heating-coil material can be used as a heating conductor. This will be comprised of at least one heating filament that runs helically around a thin core filament, and this core filament, around which a heating element is wound, then runs helically, as a heating element, around the fluid line. In this way, the length of the heating filament—and thus also its resistance—can be extended more or less as required, or as the case may be, specified as a predetermined value.
An additional aspect of the invention involves providing the fluid line with a plurality of (at least two) heating conductors and connecting them in series, in particular electrically.
According to a further alternative, a current control can be provided. For this, an electronic flip-flop or relay switch can be used, for example in combination with a temperature-dependent resistor (NTC, PTC) or a bimetal element.
Finally, the heating element can be provided with an operating voltage that is produced from the supply voltage by means of a PWM controller that is synchronized using a certain pulse-control factor (by means of pulse-width modulation) in order to adjust or regulate the heating output. Here the operating current results, in each instance, from an effective value of the synchronized, pulsed operating voltage and a respective, appropriate, temperature-dependent resistance of the heating element. Advantageously, the actual heating output of the heating element in each case can be adjusted by means of output control through variation of the PWM pulse-control factor to a predetermined standard output.
In one advantageous embodiment of the PWM controller, it is provided that in the case of a plurality of (i.e. at least two and preferably three) heat elements connected electrically in series, which are preferably assigned to the fluid line and its two line-connectors, the heating output of the individual heating elements is separately controllable and/or adjustable via the PWM controller. For this purpose, one electronic switch can be connected in parallel to each heating element, or at least to some of the total available heating elements in such a way that the heating element in question can be short-circuited in a synchronized manner by PWM actuation of the switch of the respective heating element. In addition, the entire heating-element series connection can be actuated by means of synchronized PWM.
This advantageous embodiment can be termed “intelligent control,” because the heating elements available in the heating system can largely be controlled independently of line-length and number of electric heating elements in order to optimize heating output.
In this context, it should be noted that all of the characteristics described can be employed, not only when there are “short lines,” but also independently of respective line length, for the/each line-heating element, and for each of the other heating elements for the line-connectors.